Other forms of steering with wheels
includes car-type (also called Ackerman),
tricycle, and omnidirectional. Space
doesn’t allow describing each in details.
Do a Web search if you’d like to learn
more about these other methods.
Since World War I, the tank has
become the symbol of military battles.
Little wonder that the tank design is
popular among robot builders. The same
principles that make a military tank superior for
uneven terrain apply to robots. Tracks or treads
form a wide base that enhances stability of the
vehicle. The mechanics of the treads create a
“virtual” wheel with a very large surface area
that contacts the ground.
Like the common two-wheeled bot, tracked
robots are also differentially steered. Two long
chain-like tracks (or treads) are mounted
parallel to each side of the vehicle like that in
Figure 5. A separate motor powers each track
in either direction via a sprocket; the toothed
design of the sprocket ensures that the drive
mechanism doesn’t just spin if the track gets
Though there are some hybrid vehicles that
use tracks in addition to wheels, typical tank
locomotion is provided solely by the two tracks.
The tracks are kept inline by the use of
unpowered idler rollers placed at intervals along
the sides of the vehicle.
Tracks turn by skidding or slipping, and
they are best used on surfaces such as low nap carpet or
dirt that readily allow low friction steering. When using
treads, try to pick a relatively low friction material. Very soft
rubber treads will not steer well on smooth surfaces.
No matter what the track material, it’s always best to
steer by reversing the tread directions — don’t just stop one
of the tracks. This will reduce the friction, and allow for
If both tracks move in the same direction, the robot is
propelled in a straight line forward or backward. If one
track is reversed, the robot turns (Figure 6).
This method is often referred to as tank steering or
skid steering, but at the end of the day it’s the same as
Because of the long length of the track, tank bots
don’t need a support caster or skid. The track acts as one
giant wheel, one on each side.
The main benefit of a tracked vehicle is its ability to
navigate over rough terrain. The tracks enhance the “grip of
the road” allowing the robot to travel over loose dirt, sand,
SERVO 05.2014 45
Figure 7. A handy source for robot
tracks is motorized tank toys.
Figure 8. The running gear of a
hacked tank toy may be removed
and placed on your own chassis.
This involves more work, but
provides greater flexibility in
designing your robot.
Figure 9. Unlike the common
rubber treads used in tracked
robots, plastic track links resist
stretching, so they won't pop
off as easily when the bot
makes a turn.